The proposal outlines plans to continue study of electrophysiologic properties and underlying mechanisms of ion transport in embryonic cultured chick heart cells. Besides the direct contributions of electrodiffusive and electrogenic transport, the indirect contribution of electroneutral transport to the electrophysiological properties will also be considered. Results of several experimental techniques will be integrated to establish a functional relationship between active transport by the Na-K pump and the overall regulation of ion gradient-coupled transport mechanisms, e.g., Na-Ca, Na-H exchange. Techniques include: H-ouabain binding; Na-K ATPase sensitive monoclonal antibody; K tracer kinetics; voltage-clamp; K, Na selective microelectrodes; electron microscopy with energy dispersive x-ray spectrophotometric and fluorometric determinations of Nai, Ki, Cai, Cat, Hi; electro-optical monitoring of contractions will also be carried out. Adaptive changes in the capacity of the Na-K pump will be used to study physiologic implications of active Na-K transport on cardiac cell function. The Na-K pump will be characterized in terms of its rate constant, maximum capacity, ouabain-binding properties, stoichiometry and its dependence on K and membrane potential. The long term goal is to enhance our understanding of the magnitude and interplay of ion exchange fluxes and active transport processes in regulating electrochemical and electrophysiologic properties of cardiac muscle. Adaptive changes in Na-K transport should provide new information relating in vivo reaction processes to hypokalemia as well as to therapeutic and toxic levels of cardiac glycosides.